linux-kernel - Re: [PATCH net-next] vhost: use "checked" versions of get_user() and put

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Message-ID: <CACGkMEuPK4=Tf3x-k0ZHY1rqL=2rg60-qdON8UJmQZTqpUryTQ@mail.gmail.com>
Date: Mon, 17 Nov 2025 12:32:25 +0800
From: Jason Wang <jasowang@...hat.com>
To: Jon Kohler <jon@...anix.com>
Cc: "Michael S. Tsirkin" <mst@...hat.com>, Eugenio Pérez <eperezma@...hat.com>, 
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"virtualization@...ts.linux.dev" <virtualization@...ts.linux.dev>, 
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Borislav Petkov <bp@...en8.de>, 
	Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH net-next] vhost: use "checked" versions of get_user() and put_user()

On Fri, Nov 14, 2025 at 10:53 PM Jon Kohler <jon@...anix.com> wrote:
>
>
>
> > On Nov 12, 2025, at 8:09 PM, Jason Wang <jasowang@...hat.com> wrote:
> >
> > !-------------------------------------------------------------------|
> >  CAUTION: External Email
> >
> > |-------------------------------------------------------------------!
> >
> > On Thu, Nov 13, 2025 at 8:14 AM Jon Kohler <jon@...anix.com> wrote:
> >>
> >> vhost_get_user and vhost_put_user leverage __get_user and __put_user,
> >> respectively, which were both added in 2016 by commit 6b1e6cc7855b
> >> ("vhost: new device IOTLB API").
> >
> > It has been used even before this commit.
>
> Ah, thanks for the pointer. I’d have to go dig to find its genesis, but
> its more to say, this existed prior to the LFENCE commit.
>
> >
> >> In a heavy UDP transmit workload on a
> >> vhost-net backed tap device, these functions showed up as ~11.6% of
> >> samples in a flamegraph of the underlying vhost worker thread.
> >>
> >> Quoting Linus from [1]:
> >>    Anyway, every single __get_user() call I looked at looked like
> >>    historical garbage. [...] End result: I get the feeling that we
> >>    should just do a global search-and-replace of the __get_user/
> >>    __put_user users, replace them with plain get_user/put_user instead,
> >>    and then fix up any fallout (eg the coco code).
> >>
> >> Switch to plain get_user/put_user in vhost, which results in a slight
> >> throughput speedup. get_user now about ~8.4% of samples in flamegraph.
> >>
> >> Basic iperf3 test on a Intel 5416S CPU with Ubuntu 25.10 guest:
> >> TX: taskset -c 2 iperf3 -c <rx_ip> -t 60 -p 5200 -b 0 -u -i 5
> >> RX: taskset -c 2 iperf3 -s -p 5200 -D
> >> Before: 6.08 Gbits/sec
> >> After:  6.32 Gbits/sec
> >
> > I wonder if we need to test on archs like ARM.
>
> Are you thinking from a performance perspective? Or a correctness one?

Performance, I think the patch is correct.

Thanks